The Sulfolobus Turreted Icosahedral virus (STIV) is a lytic hyperthermophile virus that infects members of the Sulfolobus family within the kingdom Crenarchaeota of the domain Archaea. The cryo-electron microscope (Cryo-EM) image reconstruction of the virus shows a striking architectural resemblance to the mammalian adenovirus, the bacteriophage PRD1, and algal virus PBCV-1. Structural studies of the major capsid proteins of adenovirus, PRD1, and PBCV-1 have shown a strong possiblity of evolutionary relationship between these viruses. This proposal sets forth structural and biophysical studies aimed in characterizing STIV and identifying whether this virus can be evolutionary related to the above mentioned viruses. The identification of a virus phylogeny encompassing the three divisions of life is exciting and unprecedented. Characterization of STIV, and Archaea infecting viruses in general, will provide substantial insight into the biochemistry of Archaea, virus host relationship and evolution, and biochemical adaptations required for survival under extreme environments. Detailed characterization of Archaea viruses may also expand our knowledge of viral evolution.